1. Technical Field
The present invention relates to an AC-DC converter of a type having an improved power factor including means for keeping a voltage in the converter at a light load from rising.
2. Description of the Prior Art
A conventional AC-DC converter has an active filter with a boost chopper as well as a rectifier for rectifying an AC input voltage and a DC--DC converter for obtaining a stabilized DC output, by which the power factor of the converter is improved.
Previously, the applicant proposed an AC-DC converter having circuit structures shown in FIG. 4 in JP Utility Model Application No. 4-61425.
In the circuit structures shown in FIG. 4, a first switching transistor Q1 and a second switching transistor Q2 are simultaneously driven by a driving signal v.sub.p from a pulse width modulation circuit (hereinafter referred to as a PWM circuit) as the same converter driving circuit.
So far control circuits or multipliers, and PWM circuits driving switching transistors by signals from the control circuits have been arranged independently to a DC--DC converter 5 and a boost chopper 4, respectively. However, because only a set of the control circuit or the multiplier, and the PWM circuit can be disposed in a circuit as shown in FIG. 4, the circuit structures can be simple and a circuit device can be small in size. Also, in comparison with an AC-DC converter having two control circuits and two signal frequencies, effects that noise generated by EMI can be highly reduced are obtained by this AC-DC converter.